Titanium alloys, although difficult to machine and fabricate, have been used successfully in ships, aircraft, and spacecraft in sheet, forged, and cast forms. One of the most notable examples of the use of titanium alloys can be found in the the SR-71 reconnaissance aircraft, formerly manufactured by the Lockheed Corporation. Titanium alloys offer high strength at elevated temperatures and a high modulus-to-density ratio. Titanium honeycomb sandwich structures, where both the face sheets and core are titanium, have particularly found wide acceptance in the aircraft industry, offering high stiffness-to-weight ratios provide by honeycomb structure, in general, as well as structural integrity at high temperatures in the 500.degree. to 600.degree. F. range.
Prior art methods of fabricating honeycomb structures have made use of diffusion bonding techniques. While diffusion bonding provides an excellent joint between the core and face sheets, it is an expensive process to use, requiring high pressures, extremely clean surfaces which are free of oxides, and the bonding process must be performed in a vacuum or, at least, in an inert atmosphere. Braising and welding are other techniques which have been successfully used to join the face sheets to the core. Organic adhesives have also been used, but elevated temperature performance is severely limited by the inability of conventional adhesives to withstand high temperatures.
Unfortunately, titanium is subjected to hydrogen embrittlement when exposed to hydrogen in either a gas or liquid form. Thus, if titanium is to be used at all in such application, a barrier must be provided between the hydrogen and titanium structure. In the past, this requirement has all but eliminated the use of titanium in applications of the nature described. The concept of using a coating over the titanium structure is an obvious solution, but there is danger of the coating becoming penetrated, and separate physical barriers add weight. Therefore, its use in hydrogen-fuel tankage has theretofore not been practical.
A load-carrying material, such as aluminum which is not susceptible to hydrogen embrittlement, integral with the titanium structure, would be ideal. But it was not thought possible to make liquid, hydrogen-fuel tankage for aircraft, space vehicles, etc., made of a honeycomb sandwich structure wherein the second face sheet is titanium bonded to a titanium honeycomb core to absorb the high temperatures, with the opposite first face sheet being of an aluminum alloy to act as a barrier between the titanium and the hydrogen using an organic adhesive. This was because of the fact that aluminum has a much higher thermal expansion coefficient than titanium and that raising the second face sheet to 600.degree. F. and above while the first (aluminum) face sheet was at temperatures below 400.degree. F. would put too much stress on the adhesive, let alone finding an adhesive that could withstand 600.degree. F. without degradation.
Therefore, it is a primary object of the subject invention to provide a honeycomb structure that is light in weight and compatible with cryogenic materials such as hydrogen fuel.
It is a further object of the subject invention to provide an adhesively bonded honeycomb sandwich structure that is capable of exposure to extremely low temperatures on the aluminum face sheet and extremely high temperatures on the titanium side without inducing unacceptable thermal stresses.
It is still a further object of the subject invention to provide an adhesively bonded honeycomb sandwich structure comprising a titanium core with a second face sheet of titanium and a first face sheet of aluminum alloy such that the structure can be used as a wall of a tank for storing cryogenic materials such as liquid hydrogen propellant.